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Old 14 May 2002, 16:05   #11
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Country: UK - England
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Going back to basics in relation to your "dream speed" of about 10 knots, the main problem will be the fact that as a rule of thumb, a boats approximate maximum displacement speed (in knots) is in the region of 1.2 or 1.3 times the square root of the waterline length (in feet). So in your case this would probably be in the region of almost 6 knots.

Above this "maximum displacement speed" the boat is starting to climb up the bow wave, if it succeeds in doing this its on the plane

Trying to get anything to go uphill requires more effort than going along the flat and as we know getting the boat running at a speed just below that where it "goes over the hump" is the least economical at which to operate, lots of throttle, huge wash and the boat trying to go uphill 10 knots is probably just about perfect in terms of being the point where your boat is just about to get over the hump and on to the plane

All in all thats a good long winded way to say that 10 knots is probably not a realistic proposition for a small auxilliary, but if you find a way to make the dream work do tell us (and probably find a way to patent the answer!)

A little auxilliary that takes you to 6 knots is going to be the most economical option.
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Old 15 May 2002, 13:42   #12
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Boat name: Prime Rib II
Make: Humber Ocean Pro
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Thanks Allan. Yes, I did say it was dream. Six will do. After all, these people with those telegraph poles in the middle of their boats spend their entire life doing six knots. The question is what size engine do I need to ensure that? Anyway, bit academic at the moment. The new boat is costing so much that oars will have to do for now. The donkey may have to wait a few months! The research can continue though!

Mike C
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Old 30 May 2002, 05:26   #13
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Mr C

My advice would be to get an Auxillary, as you can be a long way from anywhere when you realise you should have! I have had a problem which I could have easily solved with an auxilary and didn't have one. Suffice it to say it was January there was a big wind blowing (F9) and I waited hour to be rescued. I got very cold even inside my survival bag. It's definitely true what they say about the effect the cold can have

We have a 6.3 Ribtec and it's heavy and I wanted to be able to punch tide so we bought a Honda 15hp. It's great we use it as the main engine when we are in speed restricted zones so it gets some exercise.

Bring your RIB to Southampton and we will lend it to you and you can find out if a 15 hp is the right size for you. We picked it up secondhand for a grand and it still had 3.5.years transferable warranty left. NO it is not for sale.

I believe they call them donkey engines cos that's what i felt like when I realised I needed one

Cheers
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Old 30 May 2002, 08:02   #14
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Hi folks

This is very likly to make me look stupid, but its never stopped me before.

With a single engine the engine is mounted centerish on the transom. Unless you keep your aux engine on the transom you have to fit it when needed and sods law says it will be rough. A say 10Hp must weight about 50Kg and thats a lot of weight to hang over the transon, not drop and fit when the boats being thrown around.

Has anyone used or thought of two smaller engines say two 3.5's.
Light to fit and move, prob not more storeage and I bet two 3.5's are almost as cheap as one 8/10HP. Ok no gearbox etc but you only likley so set the at WOT forward and leave them there.
Also you have a more balanced drive as the engine is not off to one side, one being at each side.

What do people think I am being stupid ?. (You can be honnest)

Gary
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Old 30 May 2002, 18:08   #15
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Garry,

No I don't think you're mad. The thought did cross my mind in the past, purely from the point of view of weight & transom humping. The problem is that the weight isn't THAT much less, so two of them makes for a lot more. The Mariner 5hp for example weighs 20 kg, you have to go right down to a 3.3 hp to get lighter, (14kg). The 6 - 15 hp Mariner range all weigh 34 kg. (Source Mariner tech spec download from their website)
There is however a Tohatsu 9.8 hp which only weighs 27 kg. At the moment, this is my preferred best bet compromise..... but as I said earlier.... boat expensive. No money for a bit!

Dear Wavehumper. Yes I would love to try out your 15hp. My new boat is still a tub of resin at the moment, but I will get back to you come July/August after I take delivery.... And yes, I will not be going anywhere exotic without a spare engine. It's top of the list for buying later in the year!
PS.... How do you carry it? Kept on transom, on a bracket or humped over the back in a panic?

thanks for comments

Mike C
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Old 30 May 2002, 18:24   #16
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Where do we mount the Outboard......good question. For a while we were thinking about velcroing it on to the cowl of the main engine. It's a bit tight for width on the back of the transom.

so we mounted it on an outboard bracket for a yacht. It isn't as good a fitting as I would like so we are having a metal bracket made up to strenthen the fitting.. If neccasary I could hump it on to the transom, but with my luck the time I need it the weather won't be helping me. See you in August

Regards
Stuart
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Old 30 May 2002, 18:53   #17
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Stuart,

Thanks, August is a deal. Let me know how you get on with the strengthening. I'm looking at some fairly tough looking o/b brackets available from International marine supplies for about 100, http://www.internationalmarinesuppli...p/itm00544.htm This one is supposed to support 46 kg, but of course that's on a yacht, not bouncing around at 30 knots in a RIB. If I get the 27kg Tohatsu, it will have more of a chance of life than with a 15 hp I suppose, but not completely convinced! It may be OK with some strengthening though!

As I say, do let me know how you get on!

Mike C
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Old 31 May 2002, 14:20   #18
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Hi Mike and everyone

Yes we was thinking of say two 3.3's still 7.6HP but even at that I agree the total weight is not less, just its half the weight to fit when needed.

Not even sure myself if i would go fot it or know, time will tell.

The outboard brackets as per you web link, well I used to have a 20' yacht with a similar bracket on carrying a Johnston 8HP saildrive.
Even in the yacht I had so little faith in the bracket, the way the engine swayed and moved when it got rough I used to tie the engine to the cockpit rail, and that was on a yacht.
I think using them when you need to is OK as
a Your going to be going a lot slower
b The forward load seamed to stabilise the engine
c Part of the engine weight, the leg is in the water

I dont think I would like to risk a 10 ro 15HP on a RIB.

The other thing I have seen was again on a yacht.
Never seen one in a catalog etc but it was a sort of up and over bracket. The engine was fitted but when not is use lifted over into the boat while still on the bracket. When needed you lifted it over the transom and it locked down into the water.
When it was in the boat the engine lay flat all be upside down.
If the yachts still at the sailing club i used to go to I will try and get some pics.

Regards Gary
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Old 31 May 2002, 19:15   #19
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Thanks for that. I don't think there is an ideal answer!

On the subject of two small ones. Don't forget also that you will probably have to prop then more coarsely than you would if there were only one. This may mean that if you can't get one started, the other on it's own, (propped as a twin) may do nothing at all. Also if you need to get under auxiliary power quickly..., (Supertanker baring down on you etc) it'll take you twice as long!


Buy a sail!

Mike c
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Old 05 June 2002, 13:51   #20
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While on holiday I was thinking about aux engines, while dragging my wife and kids round to look at yet another RIB I just spotted.

One of the issues with bigger aux engines is the weight. I dont like the idea of them living on the transom but think they are too heavy to fit when needed.

However looking at A frames, could the A frame and a pully system be used to lower the engine onto the transom ? I think so.
While it would mean that you would still have to fit it when needed at least you dont have 40Kg trying to put it and you into the sea.
All you would have to do it to guide it onto the transom, the A frame taking the weight.
While I dont think a single loop A frame would stand the weight I think most double hoop ones would.

What do you think has anyone seen it done, and would it work

Regards Gary(yet another bad idea) Greenwood


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